找不到加载 JDBC org.postgresql.Driver 的类

我正在开发一个Web项目，我最近安装了postgres 9.1.1

postgresql 服务器已启动并正在运行。我可以像往常一样通过psql连接，并且所有内容都已从我从8.5创建的数据库的转储中加载并正确保存。

所以我也在这里下载了9.1 postgres版本的JDBC4驱动程序：http://jdbc.postgresql.org/download/postgresql-jdbc-9.1-901.src.tar.gz

我通过 eclipse 使用项目属性将其添加到 java 构建路径中。

这是我用来向其他类提供数据库连接的代码（即，它是一个单例，只有当现有连接关闭或为空时，我才会获得新连接，一次只能从一个对象获得）

public abstract class DBConnection {
private static Connection connection = null;

public static void connect() {
    try {
        if (connection == null) {
            String host = "127.0.0.1";
            String database = "xxxxx";
            String username = "xxxxx";
            String password = "xxxxx";
            String url = "jdbc:postgresql://" + host + "/" + database;
            String driverJDBC = "org.postgresql.Driver";
            Class.forName(driverJDBC);
            connection = DriverManager.getConnection(url, username,
                    password); //line firing the class not found exception

        } else if (connection.isClosed()) {
            connection = null;
            connect();
        }
    } catch (SQLException e) {
        e.printStackTrace(System.err);
    } catch (Exception e) {
        e.printStackTrace(System.err);
    }
}

public static void disconnect() {
    if (connection != null) {
        try {
            connection.close();
        } catch (SQLException e) {
            Logger.getLogger(DBConnection.class.getName()).log(
                    Level.SEVERE, null, e);
        }
        }
    }

    public static Connection getConnection() {

        try {
            if (connection != null && !connection.isClosed()) {
                return connection;
            } else {
                connect();
                return connection;
            }
        } catch (SQLException e) {
            Logger.getLogger(DBConnection.class.getName()).log(Level.SEVERE,
                    null, e);
            return null;
        }
    }

    @Override
    public void finalize() {
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                Logger.getLogger(DBConnection.class.getName()).log(
                        Level.SEVERE, null, e);
            }
        }
    }

}

正如我在运行项目时在标题中所写的那样，一个类要求连接到这个类，我总是得到一个类未找到异常，因为它显然无法加载org.postgresql.Driver.class驱动程序位于项目的子文件夹中~/lib/org.postgresql-9.1-901.jdbc4.jar正如我所说，通过eclipse项目属性添加到构建路径中。

我还提供了一个示例查询，让我们查看我的类访问 DBConnection 的通常行为：

public static final User validateUserCredentials(String id, String pswd) {
    Connection connection = DBConnection.getConnection();
    Logger.getLogger(Credentials.class.getName()).log(Level.SEVERE, (connection!=null)?"connection not null":"connection null");
    Statement stmt = null;
    Logger.getLogger(Home.class.getName()).log(Level.SEVERE, "validating credentials for user: username : " + id + " password : " + pswd);
    String sql = "Select * from fuser where id = '" + id + "'";
    ResultSet resultset = null;
    try {
        stmt = connection.createStatement();
        resultset = stmt.executeQuery(sql);
        Logger.getLogger(Credentials.class.getName())
                .log(Level.SEVERE, sql);
        resultset.next();
        String password = resultset.getString("pswd");
        if (pswd.equals(password))
            return new User(id, pswd);
    } catch (SQLException ex) {

        Logger.getLogger(Credentials.class.getName()).log(Level.SEVERE,
                null, ex);
    } finally {
        if (stmt != null)
            stmt = null;

        if (resultset != null)
            resultset = null;
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {

            }
            connection = null;
        }
    }
    return null;
}